As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having high energy density and high discharge voltage, into which much research has been carried out and which is now commercially and widely used.
Depending upon the shape of a battery case, a secondary battery may be classified as a cylindrical battery having an electrode assembly mounted in a cylindrical metal container, a prismatic battery having an electrode assembly mounted in a prismatic metal container, or a pouch-shaped battery having an electrode assembly mounted in a pouch-shaped case formed of an aluminum laminate sheet. The cylindrical battery has advantages in that the cylindrical battery has relatively large capacity and is structurally stable.
The electrode assembly mounted in the battery case is a power generating element, having a cathode/separator/anode stack structure, which can be charged and discharged. The electrode assembly may be classified as a jelly roll type electrode assembly constructed in a structure in which a long sheet type cathode and a long sheet type anode, to which active materials are applied, are wound while a separator is disposed between the cathode and the anode or a stacked type electrode assembly constructed in a structure in which pluralities of cathodes and anodes having a predetermined size are sequentially stacked while separators are disposed respectively between the cathodes and the anodes. The jelly roll type electrode assembly has advantages in that the jelly roll type electrode assembly is easy to manufacture and has high energy density per weight.
FIG. 1 is a vertical sectional perspective view typically illustrating a general cylindrical secondary battery.
Referring to FIG. 1, a cylindrical secondary battery 10 is manufactured by inserting a jelly roll type (wound type) electrode assembly 120 into a cylindrical case 130, injecting an electrolyte into the cylindrical case 130, and coupling a top cap 140 having an electrode terminal (not shown), for example a cathode terminal, to the upper end, which is open, of the cylindrical case 130.
The electrode assembly 120 is constructed in a structure in which a cathode 121 and an anode 122 are wound in a circle while a separator 123 is interposed between the cathode 121 and the anode 122. A cylindrical center pin 150 is disposed at the center of the winding (the center of the jelly roll). The center pin 150 is generally made of a metal material to exhibit predetermined strength. The center pin 150 is constructed in a hollow cylindrical structure formed by rolling a metal sheet. The center pin 150 serves to fix and support the electrode assembly. In addition, the center pin 150 serves as a passage to discharge gas generated by internal reaction of the secondary battery when charging, discharging, and operating the secondary battery.
Meanwhile, a lithium secondary battery has a disadvantage in that the lithium secondary battery has low safety. For example, when the secondary battery is overcharged to approximately 4.5 V or more, a cathode active material is decomposed, dendritic growth of lithium metal occurs at an anode, and an anode active material is decomposed. At this time, heat is generated from the secondary battery, with the result that the above-mentioned decompositions and several sub decompositions rapidly progress, and, eventually, the secondary battery may catch fire and explode.
In order to solve the above-mentioned problems, therefore, a general cylindrical secondary battery includes a current interruptive device (CID) and a safety vent mounted between the electrode assembly and the top cap for interrupting current and releasing internal pressure when the operation of the secondary battery is abnormal.
The above-described components will be described hereinafter with reference to FIGS. 2 to 4.
Referring to these drawings, a top cap 10 protrudes to form a cathode terminal. The top cap 10 has exhaust ports. Below the top cap 10 are sequentially disposed a positive temperature coefficient (PTC) element 20 for interrupting current through the great increase of battery resistance when the interior temperature of the battery increases, a safety vent 30 configured to have a downward depressed shape in a normal state and to protrude and rupture for discharging gas when the interior pressure of the battery increases, and a connection plate 50 coupled to the safety vent 30 at one side of the upper end thereof and connected to a cathode of an electrode assembly 40 at one side of the lower end thereof.
In normal operating conditions, therefore, the cathode of the electrode assembly 40 is connected to the top cap 10 via a lead 42, the connection plate 50, the safety vent 30, and the PTC element 20 to achieve electric conduction.
However, when gas is generated from the electrode assembly 40 due to various causes, such as overcharging, with the result that the internal pressure of the battery increases, as shown in FIG. 3, the shape of the safety vent 30 is inversed. That is, the safety vent 30 protrudes upward. At this time, the safety vent 30 is separated from the connection plate 50 to interrupt current. As a result, the overcharging is prevented from further progressing, thereby achieving safety. However, when the internal pressure of the battery continues to increase, as shown in FIG. 4, the safety vent 30 ruptures, with the result that the pressurized gas is discharged through the exhaust ports of the top cap 10 via the rupture of the safety vent 30, thereby preventing explosion of the battery.
However, this operating process is absolutely dependent on the amount of gas generated from the electrode assembly or the amount of gas discharged from the battery. If the amount of generated gas or the amount of discharged gas is not sufficient or does not increase to a predetermined level within a short period of time, a short circuit may occur late, and a thermal runaway phenomenon may occur due to continuous electric conduction of the electrode assembly. The thermal runaway phenomenon occurs or is further accelerated when the battery is in continuous electric conduction, with the result that a possibility of the battery catching fire or exploding greatly increases. Consequently, the safety of the battery is seriously threatened. Furthermore, there have continuously occurred ignition accidents of laptop computers in recent years, and therefore, the importance of the safety of the battery is being further emphasized.
Therefore, there is a high necessity for development of a cap assembly that is capable of rapidly discharging gas out of a battery when the gas is generated in the battery due to various causes, such as overcharging, with the result that the internal pressure of the battery increases.